The present invention relates to variable pitch propellers for aircraft and water craft, and more particularly to such propellers having a range of forwardly driving pitch angles and one or both of feathered and reverse pitch angles wherein moving outside of the range of driving pitch angles is undesirable except under appropriate operating conditions.
In existing variable pitch propeller propulsion systems, it is often required or desired to include feathered and/or reverse pitch angles for operations other than forward propulsion. In aircraft, for example, it is highly desirable to have a feathered pitch capability in case of engine failure, as well as a reverse pitch capability for braking in landings as well as for enhanced taxiing maneuverability. However, inadvertent or accidental activation of either of these additional pitch positions is harmful and dangerous except under certain conditions. For example:
1. Reverse pitch is generally never safe in flight, giving rise to loss of control and, particularly at high air speeds, mechanical failure; and
2. Feathered pitch is unsafe unless there is engine failure, in which case there is a danger that feathered pitch may be activated on a propeller other than that of the failed engine.
Thus there is a need for a variable pitch propeller system that provides feathered and/or reverse pitch operation automatically only under prescribed conditions.
The present invention meets this need by providing a variable pitch propeller system having feathered and/or reverse pitch operation in addition to a normal range of forwardly driving pitch positions, wherein activation of a pitch angle outside of the normal range is blocked at rotational speeds above a predetermined threshold. In one aspect of the invention, a variable pitch propeller apparatus includes an input shaft for coupling to a rotary power source on a drive axis; a plurality of propulsion blades rotatably supported relative to the drive shaft on respective blade axes, the blade axes diverging from the drive axis; an actuator having a yoke member supported relative to the drive shaft for movement therewith and coupled to the propulsion blades for rotation thereof about respective ones of the blade axes, the yoke member having a normal range of yoke positions from a high pitch position to a low pitch position, and an additional yoke position outside of the normal range, the propulsion blades having a corresponding normal pitch angle range including a high pitch angle and a low pitch angle, and an additional pitch angle outside of the normal pitch angle range corresponding to the additional yoke position; and a centrifugal lock mechanism for blocking the movement of the yoke member from the normal range of yoke positions toward the additional yoke position in response to rotational speed of the input shaft being greater than a predetermined threshold speed.
The lock mechanism can include a stop member rigidly connected to the yoke member and having a shoulder surface oriented generally perpendicular to a direction of movement of the yoke member; a mass element coupled to the input shaft for movement from an unlock position to a lock position displaced outwardly from the drive axis relative to the unlock position in response to centrifugal force, the mass element moving in predetermined relation to the drive shaft, the mass element in the lock position thereof blocking the lock member and the yoke member from moving from the normal range of yoke positions toward the additional yoke position by engagement with the shoulder surface; and means for biasing the mass elements toward the unlock position. The lock member is a first lock member and the mass element is a first mass element, the lock mechanism also including a second lock member and a second mass element symmetrically located opposite the drive axis, and wherein the means for biasing comprises a tension spring coupled between the first and second mass elements.
The additional yoke position can be any one of a feathered position opposite the high pitch position from the low pitch position, the propulsion blades having a corresponding feathered pitch angle; and a reverse yoke position opposite the low pitch position from the high pitch position, the propulsion blades having a corresponding reverse pitch angle. Also, there can be first and second additional yoke positions, on opposite sides of the normal range of yoke positions, the propulsion blades having corresponding first and second additional pitch angles. Further, the additional pitch angles can be feathered and reverse, being spaced apart by at least 100 degrees.
When the apparatus provides the first and second additional yoke positions, the lock mechanism can include the stop member having oppositely facing shoulder surfaces; the mass element moving from the normal range of yoke positions toward the first additional position by engagement with the first shoulder surface and from the normal range of yoke positions toward the second additional yoke position by engagement with the second shoulder surface; and the means for biasing the mass element toward the unlock position.
The threshold speed can be predetermined to be not greater than 1500 RPM, a preferred value being approximately 1200 RPM. The actuator can include an annular hydraulic cylinder rotatably supported relative to the drive shaft, and an annular piston sealingly movable in the hydraulic cylinder for axially displacing the yoke member. Preferably an antifriction thrust bearing is interposed between the yoke member and the annular piston, the annular hydraulic cylinder with suitable means for preventing rotation of the hydraulic cylinder during rotation of the input shaft.
The invention also provides a variable pitch propeller system including a hydraulic control unit for connecting to a fluid port of the power actuator to controllably advance the actuator piston, the controller including a housing forming a primary hydraulic cylinder; a control piston sealingly slidably engaging the primary hydraulic cylinder for forming a closed fluid cavity of variable volume in fluid communication with the fluid port when the control unit is connected to the actuator; and a lead screw rotatably supported in the housing and having a lead screw nut assembled thereto and rigidly connected to the control cylinder, the lead screw being rotatably axially supported within the housing for advancing the control piston at high mechanical advantage in response to rotation of the lead screw, the pitch of the blades being adjustable in response to rotation of the lead screw. The lead screw nut can be configured as an antifriction nut that is threadingly engaged with the lead screw by means of a plurality of rollingly interposed elements, the lead screw being axially supported within the housing by an antifriction thrust bearing.
The control unit can also include an indicator spring contact supported relative to the housing, and a plurality of indicator contact members supported relative to the lead screw nut for sequential engagement by the indicator spring contact in response to rotation of the lead screw for signaling positions of the control piston to an external device. The propeller system of claim 15, further comprising a plurality of display elements electrically connected to respective ones of the indicator contact members for activation in corresponding positions of the control piston. The display elements can be differently colored illuminators. The control unit can also have a control motor coupled to the lead screw for reversibly driving the screw in response to respective forward and reverse drive signals; respective forward and reverse spring contacts supported relative to the housing, and respective forward and reverse contact members supported relative to the lead screw nut for sliding engagement by corresponding ones of the spring contacts, each of the forward and reverse drive signals being fed serially through corresponding ones of the spring contacts and contact members to the control motor; and each of the forward and reverse contact members being foreshortened in a direction corresponding to movement of the lead screw nut in response to activation of a respective one of the drive signals for deactivating the control motor at opposite extremities of travel of the control piston.
The propeller apparatus can include two of the blade members that project from opposite sides of the hub.